The invention relates to cationic polymers derived from acrylamide obtained by the Hofmann rearrangement, also called the Hofmann degradation reaction. It also relates to the use of these polymers, essentially as dry strength agents jointly developing drainage and retention properties in a papermaking process.
Increasingly strong papers and cardboards are needed, in particular for the packaging industry.
By definition, the dry strength of the paper is the strength of the normally dry sheet. The bursting and tensile strength values conventionally give an estimate of the dry strength of the paper.
The use of water-soluble cationic polymers to improve the strength of paper is well known. Due to their nature, they can be fixed directly to the anionic cellulose and give it a cationic charge so that in combination with anionic polymers, the latter are fixed to the cellulose fibres, thereby improving the dry strength of the sheet.
The cationic acrylamide polymers known in the prior art are of various types: they are usually copolymers based on acrylamide and cationic monomers. They may also be Mannich polymers (the acrylamide polymer reacts with formaldehyde and a secondary amine and is then quaternized), glyoxalated polymers or acrylamide polymers on which a Hofmann rearrangement has been carried out.
Among all these polymers, only those obtained by the Hofmann rearrangement have specific and unique properties in terms of cationic charge density.
Until recently, only very large scale processes using, in situ, a production unit at very low concentrations of about 1% of Hofmann degradation product (EP 377313) or processes using another polymer (base of the N-vinylformamide copolymer type followed by hydrolysis) itself relatively costly (US 2004/118540), had been installed.
Only when the patents WO 2006075115 and FR 07 53347 to the Applicant appeared in early 2005 and 2007, did the production of Hofmann degradation products become industrially feasible, by proposing to use Hofmann degradation products in concentrations higher than 1%, about 3.5%, or even higher.
The action of the cationic acrylamide polymer obtained by the Hofmann rearrangement on the dry strength (tensile, bursting, internal cohesion, etc.) of the sheet depends, to varying degrees, on the quantity employed. In practice, the quantity of Hofmann degradation product introduced into the cellulose suspension is generally much higher than that of a conventional retention agent, about 800 to 1500 grams of active polymer per metric ton of dry pulp.
Whereas dry strength agents are known in the prior art to require a high dosage, no economically viable agent is yet known which, when used, optimally performs the functions both of dry strength agent and those of retention agent of fillers and fibrous materials and of dehydration accelerator, when sheets are formed without a significant development of the permanent wet strength properties.
Japanese patent application JP 61-44902 also teaches that it is possible, by reaction with polyfunctional compounds such as dihalide compounds, in predefined quantitative proportions, to crosslink polymers comprising primary amine functions in order to prepare water-insoluble resins to make them sequestering agents. A similar approach is proposed in document FR 1 437 807 to crosslink gelatine which is then treated to make it insoluble in water.
Document WO 98/18831 discloses a crosslinkable latex of which the potential crosslinking is carried out in two steps. In a first step, the polymer is partially degraded by the Hofmann rearrangement. This causes the conversion of part of the amide functions to amine functions. These functions then react together to create intra- and inter-chain bonds, that is, a process of self crosslinkage. The crosslinking is then reinforced by adding a crosslinking agent. The crosslinked polymers obtained are insoluble in water (see example 3).
The problem that the invention proposes to solve is hence to develop novel cationic polymers which have significantly better performance in terms of drainage and/or flocculation compared to the cationic acrylamide polymers of the prior art while preserving its dry strength agent properties.